The invention relates to a method for control of an exhaust-gas treatment installation of an internal combustion engine, especially of a motor-vehicle engine.
The heterogeneity of combustion processes in engines with lean mixture, especially in Diesel engines, has the effect of generating carbon particles, which cannot be burned efficiently in the engine. That is manifested, for example, by the discharge of black smoke at the outlet of the exhaust line. This phenomenon is a source of pollution that one is attempting to reduce.
The presence of a particulate filter in the exhaust line of the engine makes it possible to decrease considerably the amount of particles, dust and other forms of soot discharged into the atmosphere and to satisfy the anti-pollution standards.
Regeneration procedures make it possible periodically to burn the particles trapped in the filter and to avoid clogging thereof. The soot particles are substantially carbon-containing species, and their combustion consumes oxygen to form carbon dioxide.
Combustion of soot in the particulate filter is induced by raising the temperature of the exhaust gases within the particulate filter to a temperature on the order of 550 to 650° C., the temperature at which combustion of carbon particles retained in the filter is initiated.
Filter regeneration is controlled by a calculator, which determines whether regeneration must take place and, when it is in progress, whether it may continue.
To initiate and maintain regeneration, there is frequently used a method that consists in modifying the operating conditions of the engine to raise the temperature of the exhaust gases before they are passed into the particulate filter. These modifications often involve fuel injection, which may be delayed for at least one combustion chamber of the engine. Likewise, in certain cases, post-injection of fuel is effected during the final phase of the expansion stroke. This latter injection does not supply any additional mechanical power to the engine, but it raises the temperature of the exhaust gases.
These methods have the disadvantage of being very intrusive as regards the functioning of the engine and of significantly increasing the fuel consumption. In addition, because of a phenomenon known as dilution, they increase the fuel concentration in the lubricating oil.
Sometimes the exhaust-gas treatment installations also contain nitrogen oxides (NOx) traps, which capture the nitrogen oxides generated during combustion. During a desorption operation under reducing conditions, the nitrogen oxides are reduced to nitrogen and liberated.
Furthermore, the document US2005/0103001 A1 proposes, for an internal combustion engine, an exhaust-gas treatment installation provided with a first oxidation catalyst, followed in the direction of circulation of the exhaust gases by a nitrogen oxide trap and a particulate filter. The installation is additionally provided with a reformer, with which a reformate can be generated from the vehicle fuel. Upon command, the reformate is injected upstream from one of the aforesaid components. The reformate is rich in hydrogen (H2) and carbon monoxide (CO). When it is injected upstream from the oxidation catalyst, the reformate undergoes oxidation with the oxygen contained in the exhaust gases, generating heat. This heat is used to raise the temperature of the exhaust gases upstream from the particulate filter and to induce regeneration thereof. When the reformate is injected upstream from the nitrogen oxide trap, it is also oxidized by the oxygen of the exhaust gases, just as on an oxidation catalyst. In fact, in common with the oxidation catalyst, the nitrogen oxides trap is provided with metals capable of oxidizing hydrogen and carbon monoxide in the presence of oxygen.
If the amount of reformate injected is insufficient, the temperature attained upstream from the particulate filter will be insufficient to initiate and maintain regeneration.
If on the contrary it is excessive, the risk exists that the installation temperature will rise to a level destructive for the installation.